The present invention relates to a device, at least a portion of which is porous, for use as a prosthesis, treatment implement, and other utility, and a process of fabricating such device.
The applications and uses of synthetic biocompatible implements and devices adapted for implantation or installation in or on a human body have dramatically increased in recent years. Such implements and devices include soft tissue implants for use, for example, in breast augmentation, chin, nose, ear and other body part reconstruction and the like, nerve cuffs and scaffolds, lymphedema shunts, percutaneous skin and blood access devices, insulin cell producing implants and other cell sequestrating cage devices, artificial tendon and ligament and tendon and ligament repair prostheses, artificial heart and vascular prostheses, burn dressings, and drug infusing, releasing or delivery devices.
Oftentimes, devices of the type described above fail due to problems at the implant-tissue interface. As early as 1970, Homsey, recognized that if the implant size is in the order of centimeters, a "fibrocartilaginous" membrane or capsule isolates the implant from normal tissue. If the implant is perforated so that the interstices (pores and pore interconnections) are of the order of 1 mm or less, the implant becomes woven with the tissue, rather than encapsulated as above (Homsey, C. A., 1970, J. Biomed. Mater. Res., 4:341-356). Smooth-walled silicone breast implants fail in the order of 40-60 percent due to this thick "fibrocartilaginous" membrane (capsule) which forms around the implant creating a hard, inelastic, and often painful feeling implant. This fibrous capsule also creates other problems around implants in general because it is composed mainly of dense compacted collagen, and fibroblasts, with little or no vascularity. This leads to isolation of the implant, the implant-capsule interface, and the capsule itself from the nutrient, metabolic, and cellular advantages of good blood supply, making the implant site more prone to infection, and the infections less amenable to treatment by natural resistance mechanisms and/or blood borne antibiotics.
Porous devices known under the trade names Ivalon Sponge (polyvinylchloride) and Ashley breast prosthesis (polyurethane) were created to allow tissue ingrowth into the pores within the implant. These devices were totally porous, sponge-like devices which didn't limit the tissue ingrowth into the pores or limit implant access to bodily fluids and, with nothing to stop or control such ingrowth or fluid access, poor quality ingrown tissue, with interior calcification and hardening, resulted. Such hardening is not only uncomfortable to the recipient but also unnatural in appearance and function.
In an attempt to solve some of the above problems, at least with breast prosthesis, polyurethane foam covered silicone rubber breast protheses, both with and without a silicone shell surrounding gel, were developed, but with mixed results. For the prostheses using a silicone shell, the polyurethane foam was mechanically fixed to the shell with silicone adhesive. However, the interface between the foam and shell was weak and oftentimes resulted in delamination, sometimes with leakage of the silicone gel into the surrounding tissue. For the prostheses not utilizing a silicone shell, the gel would typically leak or "bleed" through the polyurethane cover into the surrounding tissue. Such "contamination" of the surrounding tissue with the gel caused local inflammation and gel migration to distant organs. Also, because of the three-dimensional interlocking nature of the pores within the polyurethane foam, the relatively inelastic nature of the polyurethane, and the large ratio of ingrown tissue compared to the amount of material in the foam, a "Chinese handcuff" type situation (ingrown tissue locked with foam) was created with the tissue, making it very difficult to remove or change the prostheses. Further, polyurethanes, as a class, are biologically unstable and will chemically degrade, giving rise to structure breakdown, sometimes with severe inflammation. A recent concern with such chemical degradation is the potential for release of toluene diamine (TDA), a chemical which, even in small amounts, is known to cause cancer in laboratory animals.
Previous approaches to forming porous materials (for implants or other uses) have typically included use of bubble-forming technology, sintering of metal or polymer particles into a partially fused body, expansion of polymer melts or solutions (such as used to produce Gortex), processing fibers to produce fabric felts, velours, meshes or weaves, and replicating or duplicating the microstructure of carbonate animal skeletal material. See, for example, White, R. A., Weber, J. N. and White, E. W., "Replanineform: A New Process for Preparing Porous Ceramic, Metal, and Polymer Prosthetic Materials," Science, Vol 176, pp 922-924; U.S. Pat. No. 3,890,107; Leidner, J. et al., "A Novel Process for the Manufacturing of Porous Grafts: Process Description and Product Evaluation," Journal of Biomedical Materials Research, Vol. 17, pp. 229-247 (1983). Among the problems of using bubble technology to produce porous materials is the difficulty of separately controlling pore size, pore shape, and pore interconnections. Also, the resulting pores typically include sharp edges and terminations which can cause accelerated inflammation and thus problems and/or discomfort when implanted. The sintering and polymer expansion approaches are limited to the use of only certain kinds of materials, typically metals for sintering and polytetrafluoroethylene for polymer expansion, and these may not be materials having the desired flexibility, resiliency, biocompatibility, or the like. The processing of fibers is limited because only materials which can be made into fibers can be used, and the resulting structure is basically two-dimensional. The replication of carbonate animal skeletal material, although suitable for some uses, requires milling of the material to the desired size and shape, and again the pore size and shape cannot be controlled.
Two recently issued patents, U.S. Pat. Nos. 4,859,712 and 4,889,744, disclose the use of dissolvable particles initially placed on uncured silicone, curing the silicone, and then dissolving the particles to yield a silicone product having a purportedly open-cell porous surface. The particles mentioned in both patents as the preferred solid soluble particles are crystalline sodium chloride (salt) and no other exemplary particles are identified. A number of problems or difficulties are present with the methods disclosed in the two patents including the difficulty of obtaining a completely open-cell structure since the salt particles are simply placed in the surface of the implant before curing, and then the implant is cured and the salt particles dissolved. Since many of the particles will not touch, a mostly closed-cell section is produced except at the surface layer. With this technique, it is also difficult, if not impossible, to premold the solid particles into predetermined desired shapes (since salt particles do not hold together and therefore cannot be molded), and the depth of the porous portion, and the size and shape of the structural interconnections surrounding the individual open cells cannot be controlled (since salt crystals can only be pressed into contact with one another but do not inherently stick together).
Since the treatment possible with the approaches disclosed in the above two patents is only at the surface and doesn't extend in a true three-dimensional direction, the fibrous capsule created after implantation has essentially the same thickness and density as with a smooth surfaced implant. A true three-dimensional unitary porous silicone rubber prosthesis has not been available until the technology of the present invention was developed.